1. Field of the Invention
The present invention relates to a method and apparatus for etching a substrate having a silicon surface on which a silicon oxide film is formed, and more particularly, to a method and system for etching a native oxide film formed on a silicon surface in a contact hole.
2. Description of the Related Art
A method of manufacturing a transistor, which is the most representative device embedded in an integrated circuit, includes a process of forming a contact hole in an interlayer insulating film over source and drain regions and over a gate electrode formed of polycrystalline silicon, through which an electrode is led out. The contact hole is filled with a conductive material, such as tungsten or polycrystalline silicon, in a subsequent process in order to electrically connect a silicon surface to a metal electrode.
However, it is known that if silicon is left in an atmosphere containing oxygen, a native oxide film is thinly formed on the silicon surface. Once the native oxide film is formed between the contact hole and the silicon surface, contact resistance is increased, thereby making it difficult to operate the transistor at high speed. Accordingly, it is necessary to remove the native oxide film before the contact hole is filled with the conductive material.
In order to remove the native oxide film, a method of etching a native oxide film using an etchant, e.g., radicals or ions, by converting a carbon fluorine (CF)-based gas into plasma has been disclosed in Cited reference 1 (Japanese Laid-open Patent Publication No. hei 2-119134). However, the method disclosed in Cited reference 1 has disadvantages in that an etching rate is so high that the amount of etching of the native oxide film thinly formed on the silicon surface cannot be controlled, and another silicon oxide film, for example, a chemical vapor deposition (CVD) silicon oxide film in which a contact hole is formed is fairly cut off, thereby the shape being changed. Furthermore, the method disclosed in Cited reference 1 has a disadvantage in that since silicon under the native oxide film may also be damaged, an additional etching process for removing layers damaged due to etching is required.
To overcome the disadvantages, a method of etching a thin silicon oxide film, such as a native oxide film, without converting an etching gas into plasma has been disclosed in Cited reference 2 (Japanese Laid-open Patent Publication No. 2007-180418) and Cited reference 3 (Japanese Patent No. 2501295). The method disclosed in Cited references 2 and 3 includes a process of forming a condensation layer by supplying a mixed gas including an HF gas and an NH3 gas to a surface of a silicon oxide film so that the silicon oxide film is chemically reacted with the mixed gas to alter the silicon oxide film, and a process of heating and vaporizing the condensation layer. The method disclosed in Cited references 2 and 3 is called a chemical oxide removal (COR) method. In recent years, the COR method has often been used to etch a native oxide film because the COR method has a low etching rate and can control the amount of etching of a thin silicon oxide film.
However, the COR method has disadvantages in that although the native oxide film under the contact hole is etched, contact resistance is increased. The COR method may have contact resistance that is about 101˜5 times higher than that of a wet etching method such as a buffered hydrofluoric acid (BHF) method.
Upon examining the reason why the contact resistance is increased, the inventor of the present invention has found that the contact resistance is increased due to carbon implanted into the silicon and damage to a single crystal silicon substrate due to plasma. The carbon may be carbon contained in a photoresist layer or in a CF-based gas used to dry etch the interlayer insulating film.
Since the wet etching method, such as the BHF method, cuts off the silicon to some extent, a carbon layer and plasma damaged portion of the substrate are removed, thereby reducing contact resistance. On the other hand, the COR method using dry etching does not cut off the silicon at all. A layer containing carbon is not removed, thereby increasing contact resistance.
A method of removing a high resistance layer formed of SiC or SiOx inside silicon is disclosed in Cited reference 4 (Japanese Laid-open Patent Publication No. 2003-23000). The method disclosed in Cited reference 4 includes forming a contact hole by using plasma etching and continuously performing H2 plasma treatment in the same chamber. Since the silicon is etched due to H2 plasma, the high resistance layer inside the silicon under the contact hole may be removed. However, although carbon implanted into the silicon is removed, the damaged silicon due to the H2 plasma will remain.
However, the method disclosed in Cited reference 4 additionally performs wet etching after the plasma etching, both an oxide film and a damaged layer are removed. However, since the COR method does not etch silicon at all, even though the COR method is performed after the plasma etching of Cited reference 4 is performed, the silicon damaged by the plasma may still remain. Accordingly, a process of wet etching silicon is required in addition to dry etching by the COR method.